Conventionally, in a CDMA (Code Division Multiple Access) mobile radio communication cellular system, transmission power control is carried out in both a mobile station apparatus and a base station apparatus in order to ensure adequate communication quality in the receiving-party station apparatus, and to prevent excessive interference being imposed on another station apparatus.
Within transmission power control, also, with a method whereby control is performed on the basis of a desired signal (post-demodulation desired signal power) to interference ratio (SIR value), an increase or decrease in transmission power is determined by measuring a received SIR value at a receiving station apparatus and comparing it with a preset target SIR value, and this transmission power control information is conveyed to the transmitting station apparatus.
That is to say, transmission power control information directs transmission power to be increased if the received SIR value is smaller than the target SIR value, and directs transmission power to be decreased if the received SIR value is greater than the target SIR value.
The transmitting station apparatus performs transmission power control based on the conveyed transmission power control information. The target SIR value is set so that received signals ensure the necessary communication quality over a long period, and since fluctuations of the received signals are anticipated, a margin is added to an SIR value for securing the necessary communication quality to set the target SIR value.
Here, if there are few fluctuations in the received signals, such as when both the transmitting station apparatus and receiving station apparatus are stationary, the margin will be greater than is necessary, and the transmission power of the transmitting station apparatus will be greater than is necessary.
For this reason, there is a disadvantage in that unnecessary transmission power is consumed by a transmitting station apparatus, and moreover that the capacity that can be accommodated by a system is reduced because interference imposed on other station apparatuses increases.
As a technology for overcoming this disadvantage, there has been disclosed in Unexamined Japanese Patent Publication No. 8-181653 the setting of a target SIR value by varying the margin to be added to a desired SIR value according to the amount of fluctuation of a received SIR value.
This conventional technology is effective when performing target SIR value setting when a mobile station apparatus is stationary, for example, and upward-and-downward fluctuations of the received SIR value are small.
However, the moment a mobile station apparatus begins to move from a stationary state, for instance, upward-and-downward fluctuations of the received SIR value become large. In this case, a delay occurs in updating of the target SIR value.
That is to say, the moment a mobile station apparatus begins to move from a stationary state, for instance, there is a major change in the propagation environment surrounding the mobile station apparatus from quasi-static characteristics to dynamic characteristics, with the probable result that signal quality deteriorates rapidly, the required received SIR value cannot be ensured with the target SIR value set in the stationary state, and communication quality deteriorates.
In such as case, if the target SIR value has been made on the basis of long-term communication quality, there is a problem in that updating of the target SIR value will not be in time, and it will not be possible to perform transmission power control responsive to rapid deterioration of communication quality.